Per the damage reduction universal monster ability:
Damage Reduction wrote:
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Some monsters are vulnerable to magic weapons. Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters. Such creatures’ natural weapons (but not their attacks with weapons) are treated as magic weapons for the purpose of overcoming damage reduction.
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A few very powerful monsters are vulnerable only to epic weapons... Such creatures’ natural weapons are treated as epic weapons for the purpose of overcoming damage reduction.
Some monsters are vulnerable to chaotic-, evil-, good-, or lawful-aligned weapons....A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that matched the alignment subtype(s) of the creature.
TLDR: If a monster has DR/Magic, their attacks bypass as if magic.
If a monster has DR/Epic their attacks bypass as if epic.
The odd man out is DR/Alignment, because the way I read it that section of the DR text is that it doesn't matter if the monster has DR/Alignment. Only that if the creature has an aligned keyword that their attacks bypass as if aligned.
Notably, DR for weapon damage types (slashing/piercing/bludgeoning/-) don't allow them to bypass as the same type, nor does material types (cold iron/silver/etc).
